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Copyright (C) The Internet Society (2005).

Abstract

This document describes how to use the Alternative Network Address
Types (ANAT) semantics of the Session Description Protocol (SDP)
grouping framework in SIP. In particular, we define the sdp-anat SIP
option-tag. This SIP option-tag ensures that SDP session
descriptions that use ANAT are only handled by SIP entities with ANAT
support. To justify the need for such a SIP option-tag, we describe
what could possibly happen if an ANAT-unaware SIP entity tried to
handle media lines grouped with ANAT.

SIP [3] UAs (User Agents) often support different network address
types. For example, a UA may have an IPv6 address and an IPv4
address. Such a UA will typically be willing to use any of its
addresses to establish a media session with a remote UA. If the
remote UA only supports IPv6, for instance, both UAs will use IPv6 to
send and receive media.

The Alternative Network Address Types (ANAT) semantics [7] of the SDP
[2] grouping framework [5] allow UAs to offer [4] alternative
addresses of different types in an SDP session description. The
IPv4/IPv6 dual-stack SIP UA of our previous example would generate an
offer grouping an IPv6 media line and an IPv4 media line using ANAT.
Upon receipt of this offer, the answerer [4] would accept one media
line and reject the other.

If the recipient of an offer that uses ANAT supports the ANAT
semantics, everything works as described in the ANAT specification
[7]. Nevertheless, the recipient of such an offer (i.e., the
answerer) may not support ANAT. In this case, different
implementations of the answerer would react in different ways. This
document discusses the answerer's behaviors that are most likely to
be found and describes their consequences. To avoid these
consequences, we define the sdp-anat SIP option-tag.

The sdp-anat option-tag can be used to ensure that an offer using
ANAT is not processed by answerers without support for ANAT. This
option-tag can also be used to explicitly discover the capabilities
of a UA (i.e., whether it supports ANAT).

2. Terminology

In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in BCP 14, RFC 2119[1] and indicate requirement levels for
compliant implementations.

3. The sdp-anat Option-Tag

We define the option-tag sdp-anat for use in the Require and
Supported SIP [3] header fields. SIP user agents that place this
option-tag in a Supported header field understand the ANAT semantics
as defined in [7].

Answerers without support for ANAT will react in different ways upon
receipt of an offer using ANAT. We expect that, even under the same
circumstances, different implementations will behave in different
ways. In this section, we analyze these behaviors (i.e., the
following subsections assume that the answerer does not support
ANAT).

4.1. Answerer Supports All the Network Types Offered

If the answerer supports all the network types in the offer, it may
accept the offer and establish all the media streams in it. This
behavior is not what the offerer expects because it results in too
many media streams being established. If the answerer starts sending
media over all of them, the result may be a high bandwidth usage.

The answerer may also reject the offer, because although it supports
all the network types in it, the answerer may not support them
simultaneously. The error response sent by the answerer will most
likely not be explicit enough about the situation. So, the offerer
will not understand what went wrong.

In the previous scenarios, the sdp-anat option-tag would avoid the
establishment of too many media streams and would allow the answerer
to explicitly inform the offerer that the answerer did not support
ANAT.

4.2. Answerer Does Not Support All the Network Types Offered

If the answerer does not support all the network types in the offer,
it may only establish the media streams whose address types it
understands and reject the rest. This would be an acceptable
behavior from the offerer's point of view.

On the other hand, the answerer may also reject the offer because it
contains unknown address types. The error response sent by the
answerer will most likely not be explicit enough about the situation.
So, the offerer will not understand what went wrong.

In the previous scenario, the sdp-anat option-tag would allow the
answerer to explicitly inform the offerer that the answerer did not
support ANAT.

Although RFC 3388[5] provides servers with a means to indicate
support for ANAT in an SDP description, many servers do not include
an SDP description in their responses to OPTIONS requests. The
sdp-anat option-tag makes it possible to discover if any server
supports ANAT, since they would include this option-tag in a
Supported header field in their responses.

5. Option-Tag Usage

As discussed in the previous section, the use of the sdp-anat
option-tag makes SIP messages more explicit about ANAT support. So,
SIP entities generating an offer that uses the ANAT semantics SHOULD
place the sdp-anat option-tag in a Require header field. SIP
entities that support the ANAT semantics MUST understand the sdp-anat
option-tag.

6. Security Considerations

An attacker may attempt to add the sdp-anat option tag to the Require
header field of a message to perform a DoS attack. If the UAS does
not support ANAT, it will return an error response instead of
processing the message.

An attacker may attempt to remove the sdp-anat option-tag from the
Require header field of a message. This may result in the
establishment of too many media streams.

To avoid the previous attacks, integrity protection of the Require
header field is RECOMMENDED. The natural choice to integrity protect
header fields in SIP is S/MIME [6].

7. IANA Considerations

This document defines a SIP option-tag (sdp-anat) in Section 3. It
has been registered by the IANA in the SIP parameter registry.

SIP user agents that place the sdp-anat option-tag in a Supported
header field understand the ANAT semantics.

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